(1) Field of the Invention
The present invention relates to an image forming device and an image forming method for reproducing an image using a different grayscale reproduction method for each different attribute of portions of the image, and especially to an improvement in a technology for improving the image reproducibility.
(2) Description of the Related Art
Image forming devices such as digital copiers use various grayscale reproduction methods for improving grayscale reproducibility of images.
As one example, Japanese Patent Application Publication No. 11-284850 (Patent Document 1) discloses a technology that clarifies edge portions, which are often included in character images, using the 1-pixel-cycle Pulse Width Modulation (PWM) control in which the amount of exposure is modulated for each pixel, and smoothes halftone portions (non-edge portions), which are often included in photograph images, using the 2-pixel-cycle PWM control in which the amount of exposure is modulated for each unit area composed of two pixels.
In addition to such grayscale reproduction methods, grayscale correction methods such as what is called γ correction are known as the technology for improving the grayscale reproducibility.
Here, the γ correction is a technology for correcting the grayscale so that the grayscale of the original document image (input image) is reproduced accurately in the output image. The following describes this technology in more detail. Although in the ideal, the grayscale of the input image should be in direct proportion to the grayscale of the output image, they are not in direct proportion to each other in the actuality due to some factors such as the photosensitive characteristics of the photosensitive drum, the change in the surrounding environment and the like. The relationship between the grayscale of the input image and the grayscale of the output image is therefore represented by a curve that is shifted from a straight line representing the direct proportion. The γ correction technology addresses the problem by first obtaining, as a γ curve, a curve that represents a reversed characteristics of the shifted curve, and then correcting the grayscale level using the obtained γ curve so that the shift of the curve from the originally expected straight line can be eliminated.
A typical method for obtaining the γ curve is to actually form a plurality of test patches having different density levels, on the image carrier, detect the density of each of the formed test patches, and obtain from the detection results a difference from the grayscale of the input image.
However, in Patent Document 1, different grayscale correction methods are used respectively for the edge portions and the non-edge portions. Accordingly, if the same γ correction is applied to both the edge and non-edge portions using the same γ curve, the grayscale reproducibility may be improved with respect to the non-edge portions, but may be worsened with respect to the edge portions due to the difference from the non-edge portions in the output characteristics or the like, for example.
Such a problem is not limited to the edge and non-edge portions, but may occur similarly in such image forming apparatuses that use a different grayscale correction method for each attribute of images, such attributes of images including photograph and character.